1. Field of the Invention
The present invention relates to an equipment and a method for producing polyhydroxycarboxylic acid.
2. Background Art
Polyhydroxycarboxylic acid is an aliphatic polyester produced by polymerizing hydroxycarboxylic acid. A typical example thereof is polylactide.
Examples of known methods for synthesizing polylactide include methods disclosed in JP Patent No. 3258324 B and JP Patent No. 3258662 B. In these methods, lactic acid is concentrated such that the water contained therein is reduced, followed by condensation. Accordingly, lactic acid oligomers are generated. Such oligomers are temporally depolymerized with the addition of a catalyst such as tin 2-ethylhexanoate for generation of cyclic dimers (lactides). If necessary, purification is carried out via distillation, crystallization, or the like. Thereafter, ring-opening polymerization is caused with the addition of a catalyst such as tin 2-ethylhexanoate to lactides.
In some cases, the content of water regarded as an impurity in lactic acid is approximately 10% to 15%. Therefore, in order to facilitate the initiation of esterification between lactic acid molecules, water removal is carried out in the concentration step. In this concentration step, such water is removed by heating at 120° C. to 250° C. and, if necessary, depressurization using a vacuum pump or the like.
In the condensation step, water generated in reaction of esterification between lactic acid molecules is removed by vaporization caused by heating at 120° C. to 250° C. in a depressurization environment created with the use of a vacuum pump or the like and desirably in a depressurization environment at 10 Torr or less. As a result of the condensation step, lactic acid oligomers are generated from lactic acid.
Oligomers generated in the condensation step are subjected to a depolymerization step in which oligomers come into contact with a depolymerization catalyst such as tin 2-ethylhexanoate under heating at 120° C. to 250° C. in a depressurization environment created with the use of a vacuum pump or the like and desirably in a depressurization environment at 100 Torr or less. This results in generation of lactides (cyclic ester dimers of lactic acid). In general, the generated lactides arc often in a form of a gas in the environment in the depolymerization step and can be recovered via cooling/condensation. The thus recovered lactides are separated from water, unreacted oligomers, and the like are purified via distillation or crystallization.
The purified lactides are subjected to a ring-opening polymerization step and allowed to come into contact with a ring-opening polymerization catalyst such as tin 2-ethylhexanoate and a polymerization initiator such as 1-dodecanol under heating at 120° C. to 250° C., resulting in generation of polylactide. Polylactide generated in the ring-opening polymerization step contains unreacted lactides and the catalyst. Unreacted lactides remaining in polylactide might cause deterioration in properties of polylactide and promotion of polylactide decomposition, which is problematic. Therefore, it is necessary to remove unreacted lactides. For such reason, unreacted lactides are evaporated from the surface of polylactide in a melted state by vacuum degassing treatment with the use of an agitator (this treatment is referred to as “liquid phase devolatilizing”). The thus evaporate unreacted lactides are cooled so as to be recovered.
Thereafter, polylactide in a melted state is cooled with a refrigerant such as water and formed into a desired shape such as a particle or pellet shape with a forming machine. Then, drying is carried out using hot air or the like. In many cases, drying is carried out at a temperature lower than the ring-opening polymerization reaction temperature.
As described in JP Patent No. 3258324 B and JP Patent No. 3258662 B, a dispersed in-process substance containing lactic acid and lactic acid condensates such as oligomer and lactide, which has been dispersed into the discharged vent gas during the process, is cooled and recovered using a heat exchanger or the like in the depolymerization step, the ring-opening polymerization step, the liquid phase devolatilizing step, and the drying step. However, it has been conventionally difficult to reuse such recovered substance and therefore such substance has been discarded. This has been the cause for reduction in yield. Regarding such problem, the method disclosed in JP Patent Publication (Kokai) No. 10-17653 A (1998) has been known, wherein a dispersed in-process substance is recovered using a barometric condenser that carries out direct cooling via water dispersion so as to be reused as a starting material.